1. Field of the Invention
The present invention relates to the field of flat display screens. It more specifically relates to flat screens of the type comprising a cathode with microtips for electron bombarding an anode carrying phosphor elements.
2. Discussion of the Related Art
FIG. 1 shows the functional structure of a conventional flat microtip screen in which the screen surface is formed of a glass plate supporting the cathodoluminescent anode.
Such a microtip screen is essentially formed of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed opposite a cathodoluminescent anode 5, a glass substrate 6 of which generally forms the screen surface.
The operating principle and the detail of the structure of such a microtip screen are described, for example, in U.S. Pat. No. 4,940,916 assigned to the Commissariat a l'Energie Atomique.
Cathode 1 is organized in columns and is formed, on a substrate 10, for example made of glass, of cathode conductors organized in meshes from a conductive layer. Microtips 2 are made on a resistive layer 11 deposited on the cathode conductors and are arranged within meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh, without showing the cathode conductors. Cathode 1 is associated with grid 3 which is organized in lines, an isolating layer (not shown) being interposed between the cathode conductors and grid 3. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
This device uses the electric field created between cathode 1 and grid 3 to extract electrons from microtips 2 towards phosphor elements 7 of anode 5, crossing an empty space 12. In the case of a color screen such as shown in FIG. 1, anode 5 is provided with alternate strips of phosphor elements 7, each corresponding to a color (Red, Green, Blue). The strips are separated from one another by an insulator 8. Phosphor elements 7 are deposited on electrodes, formed of corresponding strips 9 of a transparent conductive layer such as indium and tin oxide (ITO). The sets of red, green, blue strips are alternately biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed to the phosphor elements 7 facing each of the colors. The phosphor elements may also be organized in pads individualized by pixel and biased by sets of pads of same color by means of strips 9, so that the phosphor elements are still generally organized in strips. In the case of a monochrome screen, the anode is formed of a plane of phosphor elements of same color or of two sets of alternate strips of phosphor elements of same color.
The present invention more specifically relates to screens in which the anode is formed of several sets of strips of phosphor elements, or of pads of phosphor elements. Reference will be made hereafter to color screens. However, the present invention also applies to monochrome screens, the phosphor elements of which are organized in strips and to screens, the anode of which is formed of a plane of phosphor elements of same color.
Often, a screen meant to be watched from the anode, which will be called hereafter a "transparent anode screen", is associated with a filter, on the anode side, for example, a filter against electromagnetic radiation or a filter restricting the angle of sight. Such a filter is generally formed of an array of elongated parallel opaque patterns, or of two perpendicular arrays of elongated parallel opaque patterns.
The addition of such a filter to a transparent anode flat screen introduces a so-called "moire" phenomenon which is prejudicial to the quality of the display. The moire effect corresponds to a distortion (luminance and chrominance variation) of the image according to the screen region or to the angle of sight. In a transparent anode screen, the moire phenomenon is due to the presence, between the array light-emitting surface (the anode) and the display surface (the filter surface), of one or several opaque arrays, the directions of which are not perpendicular to the anode strips.
More generally, a moire phenomenon can be observed as soon as an opaque array having a direction which is not perpendicular to the direction of the light-emitting elements is located between the emissive array and the display surface, for example, if the opaque array has a direction parallel to the direction of the light-emitting elements but has a different pitch. Thus, even if the filter comprises a single array parallel to the anode strips, a moire phenomenon appears if the pitch is different, which is frequent in practice, in particular for a color screen where the width of a pixel generally corresponds to three parallel strips of the anode while the pitch of the opaque patterns of the filter is independent from the screen.
In the case of a monochrome screen with a plane of phosphor elements, the moire phenomenon appears when the displayed patterns (images) themselves form an array.
The main consequence of a moire phenomenon is that the image seen is different in luminance (and in chrominance for color screens) according to the region observed or to the angle of sight.
The moire phenomenon observed on transparent anode screens by the addition of a filter introducing an opaque array can also be observed in the case of flat microtip screens in which the cathode forms the display surface.
It is indeed preferred to make the screen viewable from the cathode to improve the light efficiency of the screen. In a transparent anode screen, a major part of the light emitted by the phosphor elements is emitted towards the cathode and is thus lost by absorption. In the case of a transparent cathode, a reflective layer may be deposited under the phosphor elements.
Thus, all the light emitted is transmitted to the observer on the cathode side.
FIG. 2 schematically illustrates an example of a so-called "transparent cathode" microtip screen, that is, a screen meant to be viewed from the cathode.
As previously, cathode 1 is made on a substrate 10, here a transparent glass substrate, of conductors 13 organized in columns. A resistive layer 11 is added on conductors 13 and microtips 2 are deposited on this resistive layer. Conductors 13 are, most often, meshed and, as an alternative, these conductors are deposited on resistive layer 11, a group of microtips 2 being deposited at the center of each mesh (not shown) defined by a conductor 13. For clarity, a few microtips only have been shown in FIGS. 1 and 2. It should however be noted that the microtips are several thousands per screen pixel.
Grid 3, formed of a conductive layer organized in rows perpendicular to the cathode columns, is deposited on an insulating layer 14 added on cathode 1, grid 3 being provided with holes 4 at the locations of the microtips.
Anode 5 is formed on a substrate 6, for example, made of glass, and is formed of phosphor elements 7 deposited on a biasing conductive layer 9 organized in strips parallel to columns 13. Referring to a screen viewable from the cathode, a reflective layer (not shown) is interposed between phosphor elements 7 and layer 9 or between substrate 6 and layer 9, to reflect the light to the cathode. This reflective function may be ensured by conductive layer 9 itself.
A problem which arises with a transparent cathode screen is that the conductive tracks of grid 3 and of cathode 1 are likely to create obstacles to the travel of light 1 to eye O of the user, even placed in front of the region viewed.
To partially solve this problem, document FR-A2,682,211 describes a solution which consists of organizing the anode in the form of parallel strips of phosphor elements parallel to the grid rows, and to provide a cathode which has no microtips above the strips of phosphor elements, the conductive grid layer also being open above the strips. The cathode conductors are not meshed but are here made in a transparent conductive layer, and only this conductive layer is present on the travel of the light above the strips of phosphor elements of the anode.
A disadvantage of this solution is that it does not suppress the occurrence of shaded areas according to the angle of sight of the observer. Indeed, the grid rows always form an obstacle to the travel of the light, since the observer cannot be strictly in front of each region viewed. Further, this solution does not enable to provide a resistive layer of homogenization of the electron emission on the cathode side. Further, the grid rows, parallel to the anode rows, introduce a moire effect.
Another solution to improve the transparency of the cathode consists of etching the resistive layer, the grid and the cathode conductors so that they have a maximum opening above the strips of phosphor elements to minimize the opaque surface on the cathode side. Although such a solution improves the brightness of the screen, it does not suppress the occurrence of the moire phenomenon due to the cathode/grid structure, which implies an array parallel to the strips of phosphor elements.
Thus, another problem which is raised in a transparent cathode screen is that an opaque array which is not perpendicular to the light-emitting strips necessarily is present between these strips and the display surface (substrate 10). Accordingly, a moire effect appears, even in the absence of a filter.
U.S. Pat. No. 5,578,225 provides, to solve this problem, a cathode and a grid which are entirely transparent, except for the microtips. The suppression of any opaque array effectively enables suppressing the appearance of the moire phenomenon since any local transparency variation is suppressed. However, this solution is, in practice, unsuited. Indeed, such a solution does not enable providing a resistive layer for equalizing the electron emission. In particular, the ITO currently used as a transparent conductive material is not sufficiently resistive to make such a layer. ITO has a resistance per square of about 20 ohms, while the resistive layer of a conventional screen generally is formed in a material having a resistance per square of about 1 M.OMEGA.. The use of ITO for the resistive layer would result in considerably increasing the distance of access to the tips by this resistive layer.
Another disadvantage of this solution is that the making of ITO cathode conductors results in a luminance degradation from one end of the cathode columns to the other due to the resistance of ITO. Indeed, although ITO is of relatively low resistivity, this resistivity is sufficient to cause a non-negligible voltage drop across each column, the columns generally being brought to a voltage comprised between 0 and 30 volts according to the brightness desired for the pixel involved. This voltage drop is not disturbing, on the anode side, due to the high biasing voltage of the anode strips (several hundred volts).
To avoid this voltage drop across the cathode columns and the grid rows, U.S. Pat. No. 5,578,225 provides an opaque lateral conductor of low resistivity along each cathode conductor and along each grid conductor. However, this solution reintroduces two perpendicular opaque arrays which then result in a new moire phenomenon.
The problems associated with the moire phenomenon described hereabove in relation with the gate and cathode arrays may also, in a transparent cathode screen, originate from filters as in a screen observable from the anode, or from additional grids constitutive of a double- or triple-grid screen.